An all-wheel drive shall mean a type of drive of a motor vehicle in which—in contrast to front or rear wheel drive—the drive force is fed to all wheels of the motor vehicle that contact the ground. Further and partly globally used designations for the all-wheel drive are AWD (All Wheel Drive) and in relation to four-wheel vehicles 4×4 (Four by Four) and 4WD (Four Wheel Drive).
The currently most frequent version consists of a single drive (mainly a combustion engine such as a gasoline or diesel engine), the power of which is distributed to a plurality of axles and wheels. All-wheel drives can be divided into two basic types from the technical viewpoint: differential-controlled or permanent all-wheel drives and clutch-controlled all-wheel drives.
Differential-controlled or permanent all-wheel drives comprise a central differential (also: longitudinal differential or center differential), which divides the drive power permanently to both axles and can be implemented as a limited slip differential.
Clutch-controlled all-wheel drives are also characterized by terms such as selectable, hang-on or on-demand. With said systems, one axle is permanently driven and the other axle is only supplied with power under certain circumstances. The clutch itself can be a simple claw clutch (manual activation), a viscous clutch, a centrifugal clutch or an electronically controlled friction clutch. The advantage of clutch-controlled all-wheel drives lies primarily in the lower system costs and in the possibility of configuring the motor vehicle with driving behavior that is typical of the marketplace as a front drive or as a rear drive in the normal mode and only to change to typical all-wheel driving behavior if required. The system with two single partial clutches and without differentials makes it possible to make the driving behavior more agile, and in the case of a front-drive vehicle to partially imitate the driving behavior of a vehicle with rear drive.
Such all-wheel drives are known for example from U.S. Pat. Nos. 5,105,901 A, 7,007,763 B2, 7,111,702 B2, 9,114,795 B2 and US 2013/0103228 A1.
As a result of the lack of differentials (central and rear axle differential) and the related automatic revolution rate equalization, the following problems can occur if both partial clutches are engaged in certain situations:
Stresses in the drive train at low speed and lateral acceleration. This presents itself for example in loud noise during parking, since the wheels have no revolution rate equalization, the shafts are stressed, and said stresses are suddenly reduced by scrubbing the tires. This results in vibrations and noise generation.
Unintended Understeering in Tight Turns:
The following only applies if the selectable axle is installed on the rear axle: The outer wheel in turns cannot produce positive torque owing to the lack of revolution rate equalization between the front axle and the rear axle. The front axle in turns runs on a smaller radius than the rear axle; the difference is greater in tighter turns. In the absence of revolution rate equalization, during engagement of the clutch of the outer rear wheel in turns the maximum speed is determined by the front axle, and thereby the maximum slip that can be achieved. If there is little slip on the main drive axle, and hence a low speed, this leads in tight turns to the rear outer wheel being decelerated when engaging the partial clutch and as a result a negative torque being produced, which in turn leads to an understeering yaw torque.
In combination with the much larger positive slip on the inner wheel in turns, here an understeering yaw torque is produced. In comparison to the outer wheel in turns, the inner wheel runs on a smaller radius. If both partial clutches are fully engaged, this is expressed in greater slip on the inner wheel, and thereby a greater torque. If the partial clutches are engaged in a tight turn here, a considerable torque can be applied, which will become linearly greater than the torque that can be applied to the outer wheel as the radius becomes tighter. This results in an understeering yaw torque, which can further boost the torque arising under point a.
Increased wear on the tires owing to the scrubbing of the tires.
Increased fuel consumption owing to the occurring negative torques mentioned under point 2.
There is therefore a need to detect the operating points at which the aforementioned problems arise and to take countermeasures so as not to adversely affect the ride comfort and the driving dynamics of a motor vehicle with such a selectable all-wheel drive without a central differential and without a rear axle differential.